JP5846080B2 - High-strength steel with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a high-strength steel material having excellent delayed fracture resistance, and particularly used in PC steel bars, spring steel materials, etc., in a range of medium carbon to high carbon having high strength and excellent hydrogen embrittlement resistance. Related to steel materials.

  Conventionally, high strength steel materials in the range of medium carbon to high carbon such as PC steel bars and spring steel materials used as prestressed concrete tension materials used in civil engineering and building structures (hereinafter simply referred to as “high strength medium to high carbon steel materials”). ") Is required to have a high-strength steel material having excellent delayed fracture resistance, and various techniques have been proposed to satisfy this requirement.

  For example, as in Patent Document 1, C, Si, Mo, Cr and B are added in an appropriate range, the maximum content of S is specified, and in the manufacturing method, in a low temperature range of 150 ° C. to 400 ° C. A steel material having a strength range of 1900 MPa or more and high corrosion resistance by tempering has been proposed.

Further, as a method for producing a PC steel rod having a tensile strength of 145 kgf / mm ² or more and excellent in delayed fracture resistance, in the hot rolling of steel, the rolling temperature Tf satisfies the condition of 700 ° C. ≦ Tf ≦ 850 ° C. After quenching with water after giving a reduction ratio of 30% or more and further tempering, the aspect ratio, which is the ratio of the length and width of the prior austenite grains, is 2 or more by performing tempering at 350 ° C. or more and 500 ° C. or less. Has been proposed (see, for example, Patent Document 2).
A high-strength PC steel rod characterized by having a ferrite having an average particle size of 5 μm or less and martensite or tempered martensite as a main structure and having an average area ratio of ferrite of 20 to 40%. perform rapid heating up _c3 temperature or higher, performed in a short time processing of the above reduction of area of 20%, a method of manufacturing by quenching from a _r3 less a _r1 temperature above (e.g., Patent Document 3 reference) it is proposed like ing.

JP 2009-257771 A JP-A-7-300652 JP 2001-294980 A

However, the steel material described in Patent Document 1 has a problem in that the delayed fracture resistance is deteriorated if there are minute defects such as chipping with a spring stone in a suspension spring or a handling rod with a PC steel rod.
Moreover, in the manufacturing methods described in Patent Documents 2 and 3, it is necessary to increase the rigidity of the rolling mill in order to perform low temperature rolling, and there is a problem that the equipment cost increases.
In view of the above, it is desired to obtain a medium to high carbon steel material that is superior in delayed fracture resistance at a lower cost than conventionally produced medium to high carbon steel materials.

Therefore, an object of the present invention is to provide a high-strength medium to high carbon steel material that is more excellent in delayed fracture resistance than conventionally produced medium to high carbon steel materials.
In particular, an object of the present invention is to provide a high-strength steel material excellent in delayed fracture resistance by controlling the prior austenite grain size in each of the central region and the surface layer vicinity region in medium to high carbon steel materials.

The present inventors have earnestly studied to improve the delayed fracture resistance of medium to high carbon steel materials. As a result, in addition to making the prior austenite grains in the center finer, it was found that hydrogen embrittlement resistance was improved by making the prior austenite grains in the surface layer elongated, thereby completing the present invention.
The present invention has been made based on further studies based on the above-described findings, and the gist thereof is as follows.

[1] By mass%, C: 0.30 to 0.80%, Si: 0.05 to 2.50%, Mn: 0.10 to 2.00%, Al: 0.005% to 0.10 %, P: 0.015% or less, S: 0.015% or less, N: 0.0100% or less, the balance is composed of Fe and inevitable impurities, and the martensite structure fraction is the area ratio When the diameter is d (mm), the prior austenite grain size in the region of d / 4 (mm) from the center is 20 μm or less, and the former austenite grain size in the region of the surface layer to 1 mm or less. Is a high-strength steel material excellent in delayed fracture resistance, characterized in that the aspect ratio in the longitudinal direction and the width direction is 1.5 or more and the tensile strength is 1400 MPa or more.
[2] Further, by mass%, one or both of Ti: 0.100% or less and B: 0.0010 to 0.0100% are contained, and the contents of Ti and N satisfy Ti ≧ 3.5N A high-strength steel material having excellent delayed fracture resistance as described in [1] above.
[3] Further, in terms of mass%, Mo: 0.05 to 1.00%, Cr: 0.05 to 1.50%, Ni: 0.05 to 1.00%, Cu: 0.05 to 1. The high-strength steel material having excellent delayed fracture resistance according to the above [1] or [2], characterized by containing one or more of 00%.
[4] The above [1] to [3], further containing one or both of Nb: 0.010 to 0.100% and V: 0.05 to 0.40% by mass% A high-strength steel material excellent in delayed fracture resistance according to any one of the above.

According to the present invention, it is possible to provide a high-strength steel material having excellent delayed fracture resistance and a tensile strength of 1400 MPa or more.
In particular, according to the high-strength steel material excellent in hydrogen embrittlement resistance of the present invention, it is possible to extend the life of prestressed concrete columns and piles, and contribute to improving fuel efficiency by reducing the weight associated with increasing the strength of suspension springs for automobiles. Therefore, the industrial contribution is very remarkable.

FIG. 1 is a hydrogen release rate curve for hydrogen analysis by the temperature rising method, and schematically shows the relationship between the temperature obtained when the delayed fracture test piece 1 is heated at a temperature rising rate of 100 ° C./h and the hydrogen release rate. It is the shown graph. FIG. 2 is a plan view showing a test piece used for a delayed fracture test of a steel material in the present embodiment. FIG. 3 is an explanatory diagram of a delayed fracture test apparatus (tester) in the present embodiment. FIG. 4 is a graph schematically showing the relationship between the rupture time and the amount of diffusible hydrogen when the constant load delayed fracture test of the test piece 1 is performed in the present embodiment.

First, the earnest examination about the improvement of the delayed fracture resistance of a medium-high carbon steel material is demonstrated, and the knowledge by the present inventors obtained by that is demonstrated.
The present inventors diligently studied various factors affecting the delayed fracture resistance of medium to high carbon steel materials having a tensile strength of 1400 MPa or more, and obtained the following knowledge. That is,
(I) If there is chipping on the surface of the steel material due to handling rods or spring stones, the delayed fracture resistance is significantly reduced.
(Ii) Hydrogen embrittlement resistance is improved when the prior austenite grains are made fine.
(Iii) Furthermore, when the surface austenite grains are elongated in the longitudinal direction, the hydrogen embrittlement resistance is remarkably improved.
(Iv) In hot rolling of steel, rolling is performed by rapidly cooling the surface layer at a temperature Tf before finish rolling of 850 ° C. ≦ Tf ≦ 1000 ° C. for a short time, and rolling only the surface layer at 650 ° C. or more and 850 ° C. or less. It becomes austenite grains in which the surface layer structure is elongated without increasing the reaction force, and fine austenite grains can be obtained by recrystallization at the center.

Hereinafter, the high-strength steel material excellent in hydrogen embrittlement resistance according to this embodiment will be described.
The high-strength steel material excellent in hydrogen embrittlement resistance according to the present embodiment is mass%, C: 0.30 to 0.80%, Si: 0.05 to 2.50%, Mn: 0.10 2.00%, Al: 0.005% to 0.10%, P: 0.015% or less, S: 0.015% or less, N: 0.0100% or less, the balance is Fe And an inevitable impurity, the martensite structure fraction is 95% or more in area ratio, and the diameter is d (mm), the region of d / 4 (mm) from the center (hereinafter simply referred to as “center The former austenite grain size of the surface layer to 1 mm or less is 40 μm or less, the aspect ratio in the longitudinal direction and the width direction is 1.5 or more, and the tensile strength is 1400 MPa or more.
First, the reasons for limiting the above composition in the high-strength steel material according to the present embodiment will be described in detail.

<C: 0.30 to 0.80%>
Since C is an element necessary for obtaining high strength, it is necessary to add 0.35% or more. More preferably, the C content is 0.40% or more.
On the other hand, if adding more than 0.80% C, the toughness decreases, so the C content is made 0.80% or less. Further, if C is added excessively, the tempering temperature for obtaining the desired strength increases, the amount of cementite (θ) generated increases, and it may be impossible to achieve both high strength and high toughness. It is preferable to make it 60% or less.

<Si: 0.05-2.50%>
Si is an element effective for strengthening steel and improving spring sag characteristics. If it is less than 0.05%, the effect on improving the strength is small, so the lower limit is made 0.05% or more. In addition, Preferably, it is 1.00% or more.
On the other hand, if it exceeds 2.50%, the ductility is lowered and the delayed fracture characteristics are lowered. In the case of a suspension spring, the upper limit is set to 2.50% in order to significantly reduce the cold coiling property. In addition, Preferably, it is 2.00% or less.

<Mn: 0.10 to 2.00%>
Mn is an element effective for improving hardenability. In order to obtain this effect, it is necessary to add 0.10% or more of Mn. However, if added over 2.00%, center segregation during casting is promoted, toughness is reduced, and delayed fracture characteristics are exhibited. descend. Therefore, the amount of Mn needs to be in the range of 0.10 to 2.00%. Preferably, the lower limit is 0.50% and the upper limit is 1.50%.

<P: 0.015% or less>
<S: 0.015% or less>
P and S are impurities, and in particular, P is an element that segregates at the prior austenite grain boundaries, embrittles the grain boundaries, and lowers toughness. The upper limits of P and S need to be limited to 0.015% or less. Moreover, it is preferable to reduce P and S as much as possible, and a suitable upper limit is 0.010% or less.

In the present embodiment, it is preferable to further add Ti and B to limit the upper limit of N.
Specifically, it contains one or both of Ti: 0.100% or less and B: 0.0010 to 0.0100% by mass%, N: limited to 0.0100% or less, and Ti and N It is preferable to control the content to satisfy Ti ≧ 3.5N.

<Ti: 0.100% or less>
Ti is an element that binds to N in steel, precipitates TiN, and fixes N, and contributes to a reduction in the amount of solute N. Thus, by reducing the amount of solid solution N by adding Ti, the generation of BN is prevented, and the effect of improving the hardenability of B is obtained.
In order to fix N in steel, it is preferable to add 3.5 N or more of Ti. However, since the effect is saturated even if adding more than 0.100% Ti, the upper limit of the Ti content may be made 0.100% or less. Moreover, in order to suppress a decrease in toughness due to the coarsening of TiN and Ti (CN), the upper limit of the Ti content is preferably set to 0.040% or less.

<B: 0.0010 to 0.0100%>
B is an effective element that contributes to improving the hardenability of the steel by adding a small amount, and has the effect of segregating to the prior austenite grain boundaries to strengthen the crystal grain boundaries and improve toughness. In particular, B is preferably added in an amount of 0.0010% or more because it has the effect of further improving toughness when added to a steel containing C and Si in the range of the present invention. On the other hand, even if B is added in excess of 0.0100%, the effect is saturated, so 0.0100% or less is preferable. A more preferable range of the B amount is 0.0010 to 0.0030%. In order to obtain the effect of addition of B, it is preferable to prevent the generation of BN by reducing the amount of dissolved N. Therefore, the limitation of the amount of N and the addition of Ti are extremely effective.

<N: 0.0100% or less>
N is an impurity and is preferably limited to 0.0100% or less. Further, the smaller the N content, the smaller the amount of Ti added, and the smaller the amount of TiN produced. Therefore, it is preferable to reduce N as much as possible, and a suitable upper limit is 0.0060% or less.

  In the present embodiment, one or more of Mo, Cr, Ni, and Cu that contribute to improving the hardenability may be selectively included.

<Mo: 0.05-1.00%>
Mo is preferably added in an amount of 0.05% or more in order to obtain the effect of improving hardenability. However, if over 1.00% is added, the alloy addition cost increases and the economic efficiency may be impaired. Therefore, the Mo content is preferably in the range of 0.05 to 1.00%, and more preferably in the range of 0.10 to 0.50%.

<Cr: 0.05 to 1.50%>
In order to obtain the effect of improving the hardenability, Cr is preferably added in an amount of 0.05% or more, but if it exceeds 1.50%, the toughness may be impaired. Therefore, the Cr content is preferably 0.05 to 1.50%, and more preferably 0.10 to 0.80%.

<Ni: 0.05-1.00%>
Ni is preferably added in an amount of 0.05% or more in order to obtain the effect of improving hardenability. However, if Ni is added in an amount exceeding 1.00%, the alloy addition cost increases and the economic efficiency may be impaired. Therefore, the Ni content is preferably in the range of 0.05 to 1.00%, more preferably 0.10 to 0.50%.

<Cu: 0.05-1.00%>
Cu is preferably added in an amount of 0.05% or more in order to obtain the effect of improving the hardenability. However, when over 1.00% is added, the hot ductility is lowered, cracking during continuous casting or hot rolling, Scratches and other defects may be promoted, and the productivity of steel may be impaired. Therefore, the Cu content is preferably in the range of 0.05 to 1.00%, and a suitable range is 0.10 to 0.50%.

<Al: 0.005% to 0.10%>
Al is a deoxidizing element. Al is an element that combines with N and precipitates as AlN. AlN has an effect of suppressing the coarsening of austenite grains in a high temperature range. However, if the Al content that suppresses the decrease in ductility of the steel wire is less than 0.005%, the above effect cannot be obtained. On the other hand, when the Al content exceeds 0.10%, a large amount of hard non-deformable alumina-based non-metallic inclusions are formed, and the ductility of the steel wire is lowered. Therefore, the Al content is preferably 0.005% to 0.10%. A more preferable Al content is 0.005% to 0.050%.

  In the present embodiment, one or both of Nb: 0.010 to 0.100% and V: 0.05 to 0.40%, which contribute to the refinement of austenite crystal grains, may be further included.

<Nb: 0.010 to 0.100%>
Nb is preferably added in an amount of 0.010% or more in order to obtain the effect of improving toughness due to the refinement of prior austenite grains, but the effect is saturated even if added over 0.100%. Therefore, the Nb content is preferably in the range of 0.010 to 0.100%, and more preferably in the range of 0.015 to 0.040%.

<V: 0.05 to 0.40%>
V is preferably added in an amount of 0.05% or more in order to obtain the effect of improving toughness due to refinement of prior austenite grains, but the effect is saturated even if added over 0.40%. Therefore, the V content is preferably in the range of 0.05 to 0.40%, and more preferably in the range of 0.10 to 0.35%.

Next, the structure of the high-strength steel material according to this embodiment will be described.
The structure of the high-strength steel material according to the present embodiment has a martensite structure fraction of 95% or more, and when the diameter is d (mm), the prior austenite grain size in the region of d / 4 (mm) from the center. Is 20 μm or less, the prior austenite grain size in the surface layer to 1 mm or less is 40 μm or less, and the aspect ratio in the longitudinal direction and the width direction is 1.5 or more.

<Martensite fraction: 95% or more>
Martensite is an essential structure for obtaining strength. In the case of the present invention, excellent characteristics can be obtained by forming a martensite structure with an area ratio of 95% or more. That is, when the area ratio of martensite is less than 95%, the amount of untransformed phases such as retained austenite phase, which does not contribute to an increase in strength, and precipitates such as carbides is excessive, and achievement of high strength of 1400 MPa or more is achieved. It becomes difficult.

  The metal structure of the steel material may be observed by SEM (Scanning Electron Microscope) after chemical corrosion using picric acid is applied to the sample. If the cross section (C cross section) perpendicular to the longitudinal direction of the steel wire rod is taken as the observation surface, a metal structure photograph of at least 5 fields of view is taken by SEM at a magnification of 500 times, and the average value of the martensite area ratio is obtained by image analysis Good.

It was found that the prior austenite grain size is a very important parameter in improving ductility and delayed fracture resistance. That is, by controlling the surface grain and the prior austenite grain size in the center, the delayed fracture resistance can be improved while ensuring ductility.
When the prior austenite grain size in the center is 20 μm or less, ductility is improved and excellent delayed fracture resistance can be obtained. However, when the prior austenite grain size in the central part exceeds 20 μm, the grain interface area decreases, the concentration of grain boundary segregation elements such as P and S increases, and the delayed fracture resistance deteriorates.
Further, if the prior austenite grains in a region having a depth of 1 mm or less from the surface layer are 40 μm or less and elongated grains having an aspect ratio of 1.5 or more, the delayed fracture resistance is remarkably improved. However, when the region where the elongated grains are formed exceeds a depth of 1 mm from the surface layer, the rolling reaction force during rolling becomes large, and it becomes difficult to control the rolling shape, which is disadvantageous in cost. Therefore, the prior austenite grain size is limited to an aspect ratio of 1.5 or more and 40 μm or less in a region having a central portion of 20 μm or less and a depth of 1 mm or less from the surface layer.

  The old austenite grains of steel are observed with an optical microscope after chemical corrosion of the sample using sodium dodecylbenzenesulfonate, ferrous chloride and oxalic acid added to the nital solution as a corrosive solution. That's fine. The cross section (C cross section) perpendicular to the longitudinal direction of the steel wire is taken as the observation surface, and at least 5 fields (1 field at the center and 4 fields at d / 4 from the center) at a magnification of 1000 times using an optical microscope, a total of 5 fields The metal structure photograph of the visual field) is taken, and the average value of the prior austenite grain size may be obtained by a cutting method.

The delayed fracture resistance described above was evaluated by the critical diffusible hydrogen content of the steel material.
First, a test piece 1 for a delayed fracture test having a shape shown in FIG. 2 was prepared, and hydrogen was allowed to enter (charge). Hydrogen charging was performed using an electrolytic hydrogen charging method, and the hydrogen level (invasion hydrogen amount) was changed by changing the charge current. Subsequently, the surface of the delayed fracture test piece 1 charged with hydrogen was subjected to Cd plating in order to prevent escape of diffusible hydrogen, and left for 3 hours at room temperature in order to homogenize the hydrogen concentration inside the test piece 1.

Thereafter, using the delayed fracture tester shown in FIG. 3, a constant load delayed fracture test was performed in which a tensile load of 90% of the tensile strength was applied to the test piece 1. In addition, in the testing machine shown in FIG. 3, when applying a tensile load to the test piece 1, the balance weight 2 is installed at one end of the lever with the fulcrum 3 as a fulcrum, and the test piece 1 is installed at the other end. I went. In addition, when installing the test piece 1 shown in FIG. 2 in a testing machine as shown in FIG. 3, it installed so that the left end of the test piece 1 might be arrange | positioned at the upper part (jig 4 side).
Then, as shown in FIG. 4, the maximum diffusible hydrogen amount of the test piece 1 that did not break after performing the constant load delayed fracture test for 100 hours or more was defined as the limit diffusible hydrogen amount. In addition, the amount of diffusible hydrogen of the test piece 1 was calculated | required from the hydrogen release rate curve of the hydrogen analysis by a temperature rising method as shown in FIG. Specifically, the delayed fracture test piece 1 was heated at a heating rate of 100 ° C./h, and the integrated value of the amount of hydrogen released from room temperature to 400 ° C. was determined by measuring with a gas chromatograph. .

  Next, the manufacturing method of the high strength steel material excellent in the delayed fracture resistance of this embodiment will be described below.

When the rolling temperature exceeds 850 ° C., recrystallization during rolling becomes prominent, and particularly the prior austenite grains in the center can be refined, so the temperature before finish rolling is set to 850 ° C. or higher. After the temperature conditions are satisfied, the surface layer is rapidly cooled by water cooling or mist cooling for a short time, and only the surface layer is lowered to a non-recrystallization temperature range of 650 ° C. to 850 ° C., and then finish rolling is performed. In this way, by controlling the surface layer temperature on the finishing rolling entry side, the prior austenite grains in the region within a depth of 1 mm from the surface layer can be elongated.
In order to make the aspect ratio of the prior austenite grains 1.5 or more, it is preferable that the area reduction rate is 20% or more.
Immediately after finish rolling, quenching is performed with water cooling or the like, and then tempering is performed at 350 ° C. to 700 ° C. in order to ensure ductility, thereby obtaining the above component composition and steel structure and a tensile strength of 1400 MPa or more. Is possible. If the tensile strength is less than 1400 MPa, it is difficult to obtain weight reduction and cost merit due to the increase in strength. Spring forming becomes extremely difficult.

By the manufacturing method described above, the high-strength steel material according to the present embodiment can be manufactured.
The above manufacturing method is only one embodiment of the present invention and is not particularly limited. That is, it is important to set the temperature before finish rolling or the reheating temperature before processing to 850 ° C. or higher, and other conditions may be determined as appropriate without departing from the scope of the present invention.

  The following examples further illustrate the present invention. Note that the effects of one embodiment of the present invention will be described more specifically with reference to examples. However, the conditions in the examples are one example of conditions used to confirm the feasibility and effects of the present invention, and The invention is not limited to this one condition example. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

  First, converter molten steel having the composition shown in Table 1 was produced by continuous casting, and if necessary, a rolling raw material of 162 mm square was obtained through a soaking diffusion treatment and a block rolling process. Next, it hot-rolled on the rolling conditions shown in Table 2, and made it the steel material shape whose diameter d is 13 mm. In addition, after setting it as the temperature before finish rolling, the steel material surface layer was rapidly cooled so that it might become the surface layer temperature in the finish rolling entering side.

Next, the steel material was straightened and formed into a straight bar, and then cut into a tensile test piece and a delayed fracture test piece shown in FIG.
The tensile test is performed in accordance with the test method of JIS Z 2241, and the delayed fracture test is performed by applying a tensile load of 90% of the tensile strength to the test piece using the delayed fracture tester shown in FIG. A constant load delayed fracture test was performed. Regarding the evaluation of delayed fracture resistance, the critical diffusible hydrogen content of the steel material was measured and evaluated as good at 1.00 mass ppm or more.

As shown in Table 1, the production No. of the present invention. The steel materials 1 to 16 have higher strength than the comparative example and are excellent in delayed fracture resistance.
On the other hand, production No. 17 is the amount of C. In No. 19, since Si is less than the range of the present invention, the hardenability is low and the strength is less than 1400 MPa. Production No. 18, 20, 21, 22, and 23 have low resistance to delayed fracture because the amounts of C, Si, Mn, P, and S exceed the scope of the present invention. Production No. No. 24 is an example in which the temperature before finish rolling is low and the central part is not recrystallized, so that the old austenite grain size in the central part is large and the delayed fracture resistance is low. Production No. No. 25 has a high surface layer temperature on the side of finish rolling and is recrystallized, so that the prior austenite grains are not elongated and the aspect ratio is low, so that delayed fracture characteristics are low. Production No. No. 26 has a low rolling reduction (area reduction), so that the prior austenite grains are not elongated and the aspect ratio is low, so that the delayed fracture characteristics are low. Production No. 27 is insufficient in strength. This is thought to be due to insufficient hardenability, and all the martensite fraction was not transformed into martensite during quenching, and some ferrite and bainite were formed, resulting in low strength. Conceivable.

1 Test piece 2 Support point 3 Balance weight 4 Jig

Claims (4)

% By mass
C: 0.30 to 0.80%,
Si: 0.05-2.50%,
Mn: 0.10 to 2.00%,
Al: 0.005% to 0.10%
Containing
P: 0.015% or less,
S: 0.015% or less,
N: limited to 0.0100% or less, the balance is made of Fe and inevitable impurities,
The martensite structure fraction is 95% or more by area ratio,
When the diameter is d (mm), the prior austenite grain size in the region of d / 4 (mm) from the center is 20 μm or less,
The austenite grain size in the region within the surface layer to 1 mm is 40 μm or less and the aspect ratio in the longitudinal direction and the width direction is 1.5 or more,
A high-strength steel material excellent in delayed fracture resistance, characterized by having a tensile strength of 1400 MPa or more. Furthermore, in mass%,
Ti: 0.100% or less,
B: 0.0010 to 0.0100%
One or both of
Ti and N content is
Ti ≧ 3.5N
The high-strength steel material having excellent delayed fracture resistance according to claim 1, wherein: Furthermore, in mass%,
Mo: 0.05-1.00%,
Cr: 0.05 to 1.50%,
Ni: 0.05-1.00%,
Cu: 0.05 to 1.00%,
The high-strength steel material excellent in delayed fracture resistance according to claim 1 or 2, wherein one or more of them are contained. Furthermore, in mass%,
Nb: 0.010-0.100%
V: 0.05 to 0.40%
One or both of these are included, The high strength steel material excellent in the delayed fracture resistance of any one of Claims 1-3 characterized by the above-mentioned.

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